LINAC radiation therapy systems are well known and widely used for radiation treatment of lesions at various locations in the body of the patient. The LINAC device rotates around the body part and applies a variable intensity of radiation to radiate the lesion as accurately as possible.
Typically it is known that an initial CT scan is carried out to plan the treatment using the attenuation data available from the CT image. The CT imaging is effected at a separate location and it is necessary to register the image of the CT scan with the treatment device to use the attenuation data. For this purpose a head-neck extension board is located on the LINAC's table and on the CT table with the same locking space. Also the patient's head is located on the head neck extension board by a mouldable thermoplastic mask. This is intended to ensure that the patient is located in the same location as the patient moves from the CT image system to the LINAC treatment system.
Thus the head-neck mask is the common reference frame between the two devices. Moreover, CT compatible fiducials are provided on the head-neck mask.
A coned beam CT (CBCT) that is integrated with LINAC's structural frame and has the same point of rotation as the LINAC is used to correlate 3D low resolution CT images from the CBCT with the ones obtained from the CT imaging system and utilized for the planning phase. If the correlation of the bony areas of the patient near the area of treatment coincide, then the planning treatment with the LINAC can proceed. The common point between the CBCT, the regular CT scanner is head-neck mask that conforms to the exact shape of the head and neck by inserting the mask into a hot water solution and then place it firmly over the head and neck region. This conformance the mask is unique to every individual treated and accompanies the individual to every stage of the radiation therapy form the acquisition of the CT image, to the planning stage to the treatment with LINAC.
The center point of interest on the CT scanner is determined by a set of 3 laser beams that are aligned with the three fiducials on the mask and coincide at a point which is the same rotational point used on the CBCT and LINAC set. Thus by identifying this point it is able to superimpose the images from CBCT, to previously collected images from CT and ensure that the patient is positioned correctly for the LINAC treatment.
This system is widely used and has been found to have a relatively low accuracy which can often be as poor as 2 cms so that the treatment has to follow a very conservative course to avoid radiation to healthy tissue. Thus a higher number of treatments with a lower dose of radiation is typically necessary than would theoretically be necessary.
It is also appreciated that MR imaging provides a much more suitable image of the lesion. For this reason one development in this area is to provide a combination of an MR imaging system with a LINAC so that the patient can remain in the MR imaging system location when the LINAC treatment is carried out. Philips Medical Systems (see attached paper).
With MRI, a high field magnet, typically superconducting, is arranged in a torus configuration and with the patient lying down inside the magnet on a table where the magnetic field allows a pulsed and sequenced magnetic and EM field to probe the body to produce images, which allow the trained radiologist to determine with high probability the anatomy of the patient. MRI techniques are very good at detecting the anatomical location of different diseases, for example, tumours.
The magnet for highest field strength is generally cylindrical with the patient lying along the bore but other magnet shapes and arrangements are used in lower strength configurations.
Most MR imaging is carried out on a diagnostic basis where the patient is placed on a diagnostic table which moves into the bore and carries the patient to a required position. In these cases the patient is normally conscious and thus can be moved to the required location on the table so that, when the table enters the bore, the part of the patient to be imaged is located relative to the table so that it enters the bore at the field of view of the MR system, generally at the center of the bore on the axis. In most cases therefore the table is movable only longitudinally of the bore to move the patient into and out of the bore.
The table also is prevented from any significant movement relative to the bore since it includes mechanical structures which fill the available part cylindrical area underneath the surface of the table.
In U.S. Pat. No. 5,735,278 (Hoult et al) issued Apr. 7, 1998, disclosed a medical procedure where a magnet is movable longitudinally relative to the table and to the patient on the table. The moving magnet system allows intra-operative MRI imaging to occur more easily in neurosurgery patients, and has additional applications for liver, breast, spine and cardiac surgery patients. The table is set up for the required surgery and the magnet is moved into place over the table to effect the imaging. The table can tilt about a transverse axis and rotate about a longitudinal axis.
Related disclosures are made in U.S. application Ser. Nos. 12/333,032 filed Dec. 11, 2008 (Publication 20090306495 Dec. 10, 2009) and 12/420,859 filed Apr. 8, 2009 (Publication 2009/0306494 Dec. 10, 2009) by the present Assignees, the disclosures of which are incorporated herein by reference, which correspond to PCT Applications CA/2009/000672 and CA/2009/00673 filed May 25, 2009, to which reference may be made for further detail.
U.S. Pat. No. 7,319,325 (Petot) issued Jan. 15, 2008 and assigned to Philips claims the use of a position sensor to provide feed back to the drive system of a table within the bore of an MRI magnet. The movement as disclosed appears to be simply linear movement along the bore.
U.S. Pat. No. 7,486,076 (Nagao) issued Feb. 3, 2009 and assigned to Ricoh or Hitachi discloses moving the table based on information relating to the difference between the actual position and a desired position. Movement in a second direction at right angles to the longitudinal direction is possible during whole body scanning.
U.S. Pat. No. 6,822,447 (Yamagata) issued Nov. 23, 2004 and assigned to Toshiba discloses moving the table in two directions to move the point of interest to the required imaging position in the magnet.
U.S. Pat. No. 6,045,262 (Igeta) issued Apr. 4, 2000 and assigned to Hitachi discloses a table which is used in X-ray and MRI which can be moved in two directions.
Published US Patent Application 2005/0154291 (Zhao) published Jul. 14, 2005 discloses a table which is used in MRI which can be moved in two directions and tilted during imaging in the magnet.